1. Field of the Invention
The present invention relates to a nuclear magnetic resonance diagnostic apparatus (to be referred to as an "NMR diagnostic apparatus" hereinafter) which utilizes the magnetic resonance phenomenon so as to noninvasively measure information as to the spin density and the relaxation time of a specific atomic nucleus or proton within a selected sectional slice plane of an object to be examined, e.g., a patient, for which tomographic imahe is taken, and to display information for medical diagnosis in the form of tomographic images. The invention relates, more particularly to automatic tuning circuits for the NMR diagnostic apparatus.
2. Description of the Prior Art
The NMR diagnostic apparatus as mentioned in the preamble is known from, e.g., U.S. Pat. No. 4,254,778 issued on Mar. 10, 1981.
In general, a reception section in the known NMR diagnostic apparatus, which receives NMR signals including not only FID signals but also echo pulse signals, is comprised of saddle-shaped coils, so-called "probe head coil", and an internal capacitor which constitute a resonant circuit for a receiver circuit. The NMR signals are excited by the NMR phenomenon from an object, e.g., a patient around which the probe head coil is arranged. Generally speaking, since those NMR signals are very weak in the electromagnetic field strength, it is necessarily required such a resonant circuit having an extremely high "quality factor (Q)" so as to pick up the NMR signals in a high efficiency. As a result, the resonant characteristic curve of the above-mentioned resonant circuit is very sharp. Accordingly based upon a very small variation in the capacitance component of the resonant circuit, amplitudes of the received NMR signals are greatly changed. In other words, the sensitivity of the receiver varies considerably. Meanwhile there exists a stray capacitance between the patient and the probe head coil, which is different from respective patient himself, and from respective body portion for diagnosis purposes. Consequently it is necessary to take such a diagnostic preparation in the individual patient that the capacitance of the internal capacitor is varied so as to tune to each resonant frequency the resonant circuit to which the individual patient is arranged.
It is therefore an object of the present invention to provide an NMR diagnostic apparatus having an automatic tuning circuit by which in advance of an NMR signal acquisition for diagnosis purposes, the signal reception unit can be automatically tuned.